Required tools

Creating a basic denim texture

Create a diagonal line pattern

First of all we are going to create a new pattern that consists of a diagonal line.

Create a new square transparent image of 6x6 pixels:

Choose the menu option File -> New and a window will pop-up.

Set Width and Height to 6 and select pixels as the measurement unit

Expand the Advanced Options part and select Transparency from the "Fill with" drop down list, as shown in picture Adding a transparent 6x6 image.

Press the + key several times to zoom the image you've just created until it is big enough for you to work in it.

We are going to paint now one pixel at a time. For doing this:

Select the pencil tool from the toolbox

Keep the opacity at 100%

Select the Circle (01) brush, and keep its scale at 1.00, as shown in picture Pencil settings

Set the foreground colour to black and paint the pixels in the diagonal of the image, from top left to bottom right as shown in picture Diagonal line pattern.

Now we are going to save the image as a pattern:

Select the menu option File -> Save as

Enter a name for the file with extension .pat (for example, DiagonalLine.pat)

Save it to the folder \Program Files\GIMP-2.0\share\gimp\2.0\patterns\

After pressing the Save button a window will pop up asking for a description. Enter a suitable one like Diagonal Line (this desription is what you will see in GIMP when browsing your installed patterns, so enter something meaningful).

Next step is to make GIMP to load the new pattern we've just created:

Select the Bucket Fill tool from the Toolbox as shown in picture Adding the pattern to GIMP.

Select the Pattern Fill option

Click on the pattern little square image and a window will be displayed showing all installed patterns.

Click on the little bucket icon at the bottom right corner of the available patterns window and this will open the Patterns dialog as shown in picture Patterns dialog.

Click on the Refresh patterns button (the two circular arrows icon) and wait while GIMP adds all the new patterns it finds to the list.

Close the Patterns dialog and now the Diagonal Line pattern will be available.

Create the basic texture

RGB noise window

Fill top layer with diagonal line pattern

Bump map window

Basic Denim texture

Create a new square image. For this tutorial I've created a 512x512 image, but you can use any size you want.

Fill the image completely with a dark blue colour. For this tutorial I've used the 3b3b85 color.

Add noise to the image:

Select the menu option Filters -> Noise -> RGB Noise.

A window will pop-up like the one in picture RGB noise window. Uncheck the Independent RGB flag and set a high value in the noise boxes (I've left it to the edfault value of 0.20 but you can try different values)

Press OK to close the window and wait until GIMP has finished adding the noise to your image.

Now add a new transparent layer on top of the blue one:

Select the menu option Layer -> New Layer

Add a name for the new layer, select Transparency on the pop-up window and click OK

Note: To see and work with the different layers that you have in your image:

Select the menu option Windows -> Dockable Dialogs -> Layers '

This will open a window with the layers stack for your image as the one shown in picture Layers stack'

To work on one of the layers, select the corresponding layer in the stak and then work in GIMP main edition window: all you do (painting, erasing, resizing, etc) will be done on the selected layer.

To hide/show one of the layers, click on the eye icon at the left of each of the layers in the layer stack.

Fill the new layer with the Diagonal Line pattern we created before:

Select the Bucket Fill tool from the Toolbox as shown in picture Adding the pattern to GIMP.

Select the Pattern Fill option

Click on the pattern little square image and a window will be displayed showing all installed patterns.

Select the Diagonal Line pattern you've just created from the list.

Fill the transparent layer with the pattern and your image will now look like the one in picture Fill top layer with diagonal line pattern.

In the Bump map box, at the top, select the layer that you've filled with the Diagonal Line pattern.

Check the Invert bumpmap checkbox.

You can change the values of the properties, and see how if affects the original texture in the Preview window (check the Preview box to see this changes). In this case I find that the Sinusoidal map type gives better results, but you can try the other values and see which one looks best for you.

Once you've got the results you want, click OK.

Delete the top layer that you filled with the Diagonal Line pattern as we only needed it to use it with the bump map filter.

Now your image looks like the one shown in picture Basic Denim texture.

Note: If you want the diagonal lines to be more deeply marked in your texture:

Don't delete the layer that you filled with the Diagonal Line pattern. Select this layer in the layer stack and change its layer mode from Normal to Burn and the Opacity value from 100 to 20 (you can play with other blend modes and values to see which ones fulfill your needs)''

In the main edition window, select the menu option Layer -> Merge Down'